1. Field of the Invention
The present invention is related to Magnetic Amplifying Magneto-Optical Systems (MAMMOS).
2. Description of the Background
Magnetic Amplifying Magneto-Optical System (MAMMOS) is a relatively new Magneto-Optical (MO) readout technique proposed by Hitachi-Maxell which may be used to push optical data storage densities to 100 Gb/in2 and beyond. The media consists of two layers of magnetic material as shown in FIG. 1. The bottom (storage) layer is used to store information. The top (readout) layer is used to readout information. In MAMMOS, the optical read back signal is amplified by expanding the magnetic domains in the readout layer to fill or partially fill the laser spot. A non-magnetic layer, used to provide isolation, separates the two layers. FIG. 1 depicts the magnetic structure along with the external stimulus. The compositions of the storage layer and readout layer are chosen such that a laser heating the media causes a decrease in the coercivity of the readout layer. That decrease is sufficiently large such that the combination of an external field and the stray field from a domain in the storage layer is enough to overcome the coercivity and nucleate a domain in the readout layer. Using this method, when a domain is present in the storage layer, it can be duplicated and expanded in the readout layer.
To write to the medium, a laser is focused onto the surface to heat the media and reduce the coercivity of both layers locally. An external field is then applied, and the magnetization of the storage layer is switched into the direction of the external field in the locations where the field exceeds the coercivity.
During readout, the same laser beam is used, but the laser power is reduced to a level that will not alter the storage layer. Readout is accomplished by utilizing the polar Kerr effect. This effect causes the polarization of the reflected beam to be altered by the magnetization of the surface from which it is reflecting, in this case, the readout layer. By adjusting the readout layer properties appropriately, the readout layer can be used as a mask to select bits that are smaller than the laser spot from the storage layer.
When the laser beam heats the readout layer, the coercivity drops locally resulting in a coercivity profile with a minimum at the laser location. As mentioned, the magnetic properties of the two layers are adjusted so that when an external field is added to the field created by the data in the storage layer, the combined field is slightly larger than the coercivity of the readout layer at its lowest point. In FIG. 2, the lower curve is a plot of the fields inside the readout layer down the center of the track created by the data in the storage layer. The upper curve represents the coercivity of the readout layer after heating by the laser beam, also down the center of the track.
FIGS. 3A–3C illustrate the coercivity profile and the external plus stray fields at consecutive points in time during a MAMMOS data readout process. To readout data, an external field is applied at the frequency of the passing data recorded in the storage layer. Thus, as the coercivity profile moves across the bits in the storage layer, the total external field will reverse sign with position of the laser. The laser power and external field are adjusted such that the sum of the external field and stray field from the storage layer causes a domain to nucleate in the readout layer when a domain is present in the storage layer. In the case where no domain is present in the storage layer, these fields are insufficient to nucleate a domain in the readout layer. In typical MAMMOS, each of the domains in the storage layer represents a bit of data.
In FIG. 3A, the media is being probed for information in the storage layer at a location with no domain. In the figure, the external field has been switched positive at a bit location. Because there is no domain, there is no intersection between the two curves, nucleation cannot occur in the readout layer, and there is no change in the readout signal. After probing at this location, the external field is reversed or turned off as the laser moves toward the next bit location as depicted in FIG. 3B. When the laser arrives at the next bit location, the external field is turned on again to test for a domain as shown in FIG. 3C. In that case, there is a domain present in the storage layer. As shown in the figure, the two curves overlap, causing a domain to nucleate in the readout layer where the sum of the external and stray fields is larger than the readout coercivity.
Once a domain is nucleated in the MAMMOS media, it is immediately expanded due to forces on the domain wall. The domain expands to a size that fills most or all of the laser spot, resulting in a large readout signal. After passing over the domain, the field is reversed again, collapsing the domain in the readout layer and lowering the readout signal. If no domain has been recorded, the direction of magnetization of the storage layer is the same as the readout layer, no domain will be nucleated in the readout layer, and no MAMMOS signal will be detected.
The readout signal in MAMMOS is essentially digital, going ‘high’ at each potential bit location only if there was a domain present in the storage layer. FIG. 4 shows the external field and expected readout signal along with the data stored in the storage layer. At each bit location, the field is increased, and a high readout signal is expected to occur each time a domain was recorded.
The resolution limit to MAMMOS is set by the width of the tip of the thermal profile used as the detector as well as the method by which the data was recorded. In the ideal case, there is no noise in the system, media properties are constant, and there are no variations in laser power, focus, etc. In that case, the size of the tip used for readout can be made infinitely small. In reality, all of the above factors will limit how low the threshold can be set, and thus the resolution of the system. In addition, when pseudo-random data is written, neighboring domains in the storage layer will cause the maximum amplitude of the field in the readout layer created by a domain in the storage layer to shift. These shifts in amplitude limit the height and thus the width of the thermal profile that is needed for detection.
To achieve a smaller thermal profile, and thus a higher resolution, it is important to have good control over the laser power, have a small laser spot, have a uniform medium, and accurate focus. To further increase resolution, researchers have attempted to make the thermal profile steeper by adding heat sink layers into the medium. This will work to some degree to increase the resolution of MAMMOS. Another approach is to decrease the size of the domains in the storage layer. This has the effect of making the width of the stray field from the storage layer smaller and decreasing the neighborhood effect by moving domains in the storage layer further apart, thus increasing the capacity of the system. Using some of these techniques, Hitachi-Maxell has achieved information storage with a bit spacing of about 100 nm.
The need exists for a MAMMOS that has a large storage capacity and is independent of fluctuations in laser power, media, and the like.